1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle (HEV), and, in particular to a method for performing transmission input torque modulation during a change to a lower gear.
2. Description of the Prior Art
In a conventional vehicle equipped with a transmission that produces step changes among gear ratios, the driver can experience driveline disturbances during a gear shift. The driveline disturbances occur due to the acceleration and deceleration of the engine and transmission component inertias, which produce an inertial torque during the gear shift. In the case of an upshift, the transmission output torque increases during the ratio change, i.e., inertia phase of the gear shift, as a result of the engine speed changing.
This output torque disturbance is directly felt by occupants of the vehicle and affects shift quality. The level of output shaft torque disturbance increases with the speed of the upshift since engine deceleration is greater with faster gear shifts. By reducing engine torque produced during the upshift, inertial torque can be offset and the output shaft torque increase can be minimized, thereby improving shift quality. The method of reducing engine torque produced during the upshift is referred to as “input torque modulation” control.
In the case of a downshift, the transmission output torque decreases during the ratio change phase as the engine and transmission components accelerate to the synchronous speed for the lower gear. Moreover, during the torque transfer phase of the downshift, the transmission output torque can spike near the completion of the downshift as the engine accelerates. The drop in output torque during the ratio change is directly felt by the vehicle occupants and can give the sense of an acceleration discontinuity as the downshift is performed. The output torque spike at the end of the downshift can affect shift quality and produce a feeling of a rough shift. Furthermore, the level of output shaft torque drop and spike near the end of the downshift will increase in proportion to speed of the downshift. The engine combustion torque can be reduced near the end of the downshift in order to reduce the engine's acceleration as the shift ends.
By using input torque modulation, the engine combustion torque can be reduced near the end of the downshift in order to reduce the engine's acceleration as the shift ends. As a result, the transmission output torque spike can be minimized and avoided, thereby reducing the shift disturbance.
The level of input torque modulation is calibratable as a percent reduction from the current engine torque and can vary with the shift progression. Moreover, input torque modulation is triggered as a function of the shift progression by monitoring the ratio change or directly monitoring the speed sensors, e.g. the turbine speed.
In conventional vehicle applications, limitations and problems with input torque modulation during gear shifts include limited engine torque reduction authority due to constraints, such as emissions; delayed engine torque response to torque modulation requests, further degrading shift quality; and poor fuel efficiency, since spark retardation is commonly used for achieving torque modulation requests.